This invention in general relates to the field of optical recording and in particular to a system and method for initializing and formatting phase change recording media.
The use of optically recordable phase change material is known in the art. For example, U.S. Pat. No. 3,530,441 issued to Ovshinsky discloses a semiconductor material disposed on a substrate. Data is recorded by providing radiant energy to discrete portions of the semiconductor material so as to change a physical property, and data is read by sensing a difference in the material""s physical properties, such as surface reflectance. A phase change material such as TexSbyGez (Tellurium Antimony Germanium) (xe2x80x9cTAGxe2x80x9d) can be used as the active layer of a recording medium. Information is recorded by reversibly forming amorphous bits in the crystalline TAG recording layer. Other chalcogenic alloys such as Texe2x80x94Snxe2x80x94Ge can also be used as the phase change material. Before data can be recorded on this material, it must be initialized into the more highly reflective crystalline c-phase. The recording layer, which is initially amorphous, can be selectively converted into a crystalline layer by a temperature treatment. The energy to heat the recording layer is provided by laser beams in industry standard initialization devices. A material having a low birefringence, such as polymethyl methacrylate (PMMA), is typically used as a protective layer and substrate for the recording medium, where the light is transmitted through said substrate. On the other hand, the advent of near-field optical recording places the recording layer on top of this substrate, with a much thinner protective layer applied.
The initialization time for a 1 mw-5 mw laser with a 0.35 micron spot diameter, which converts the phase change material into a crystalline structure is approximately 50-300 nanoseconds. The time to initialize an entire recording medium is the time required for converting the initially amorphous material to a crystalline phase. Even with the use of a powerful laser as the radiation source, the initialization time can be relatively long. For example, the initialization time crystallizing the phase change material in an entire CD-RW disk recording medium consisting of a TAG alloy may be on the order of 15 seconds.
The linear speed of the rotating element depends on the type of information which is recorded, for example audio information, video information or data information. For example the typical linear speed is from 1 to 20 m/sec. During the data recording process, the crystalline recording material is heated above the melting temperature by exposure to radiation, typically from a laser source. This produces a localized melting area which cools so rapidly that no crystallization takes place, and a localized amorphous information area is formed. If the pulse time of the recording laser beam is on the order of from 20-100 ns, the localized amorphous information areas have small diametrical dimensions of at most one or a few micrometers so that a high information density is obtained.
In the erasing process the amorphous information areas are exposed to radiation, comprising an erasing spot, by which the amorphous area is heated to a temperature which is slightly lower than the melting-point of the material in the crystalline phase but is higher than the glass transition temperature. The amorphous area becomes less viscous and returns to the thermodynamically more stable crystalline state. It is necessary that the material be kept at the elevated temperature for a minimal period of time (the erasing time) in order to complete crystallization in the area.
A further time consuming problem with known initialization methods is that each individual disk must be loaded onto the apparatus, brought up to speed, initialized track by track, spun down, and unloaded. This process consumes additional handling time. Another requirement of the initialization process is the need for continuous laser power to initialize an entire CD disk. In addition to the time required, another problem in initializing recording media is uneven initialization and cracking or other destruction of the phase change material layer.
The limitations of the existing art highlight the need for a system and method to mass produce and initialize phase change recording media more reliably, efficiently, and with greater flexibility. While the art describes a variety of recording media with associated initialization techniques, there remains a need for improvements that offer advantages and capabilities not found in presently available initialization devices. It is a primary object of this invention to provide such improvements.
One object of the present invention is an improvement in the speed with which phase change recording media can be initialized. A further object is to minimize the handling time for an individual disk in the initialization process by initializing the media with a single exposure without having to initialize each track of a rotating disk. Another object is to limit the energy exposure time and intensity to the media, such that there is no surplus energy after initialization that can form deleterious surface in the form of cracking. Other objects include but are not limited to obtaining higher uniformity, compatibility with web-processed media including tape, and providing the ability to pre-process the media with data or encryption. Finally, it is an object of the invention to provide a whole-field optical initialization, rather than a serially exposed initialization.
The present invention is a system for initializing a recording medium comprising phase change material having at least an amorphous state and a crystalline state. The transformation between the states is effected by incident radiation. The radiation can be generated from a white light source, from a flash lamp, or from coherent radiation of wavelength xcex. It is another object of the invention to provide for a method initializing phase change recording media more rapidly than conventional means by avoiding the extensive handling of individual disks. A further object of the invention is to initialize the phase change material uniformly without cracking or destroying the phase change layer.
In accordance with the present invention, the foregoing objectives are achieved with a system using radiation from a flash lamp, a white light source, or a combination of coherent radiation of one or more wavelengths, converting the radiation into a substantially uniform field of radiation over a predetermined area covering the dimensions of the recording medium containing the phase change material, and controlling the power of said uniform field of radiation to produce a surface power density and exposure duration sufficient to convert the phase change material into the crystalline state without cracking or otherwise destroying the material.
These objectives are further achieved in accordance with the present invention with a method of initializing large areas of recording surface in more than one exposure. A further object is the ability to initialize phase change recording media on a continuous web of phase change material applied to a substrate. Yet another object of the invention is the ability to record information on the medium at the time of initialization. Still another object is to expose the media in such a way that full initialization is not reached such that the media is seeded with crystallization predisposition sites that may be subsequently developed to full initialization with another exposure or may facilitate writing smaller marks in the media drive. Other features of the invention will be readily apparent when the following detailed description is read in connection with the drawings.